The use of adsorption techniques to separate a gaseous component from a gaseous stream was initially developed for the removal of carbon dioxide and water from air. Gas adsorption techniques are now conventionally employed in processes for the enrichment of hydrogen, helium, argon, carbon monoxide, carbon dioxide, nitrous oxide, oxygen and nitrogen. Gas enrichment utilizing at least one, typically two, adsorption vessels in a cycling pressurized relationship is commonly referred to as pressure swing adsorption (PSA).
A typical PSA process for enriching a gas, for example nitrogen from air, employs at least one, generally two or more, adsorption beds filled with molecular sieve material, each being subjected to two or more, generally four, distinct processing steps in each cycle. In a first step of the cycle, one adsorption bed is pressurized with concomitant nitrogen production while the other bed is regenerated, such as by venting. In a second step, often referred to as pressure equalization, the adsorption beds are placed in fluid communication, thereby being brought to an intermediate pressure. In a third step, the first adsorption bed is regenerated, sometimes with a countercurrent flow of product-quality gas to enhance the regeneration (referred to as "purge"), while the second bed is pressurized with concomitant nitrogen product. The last step of the cycle is pressure equalization between the beds. During such pressure swings, pressure conditions in the adsorption beds typically vary from about 15 psig to 120 psig in a process employing carbon molecular sieves for nitrogen production and somewhat lower pressure ranges in processes employing crystalline zeolites for producing oxygen.
Although pressure swing adsorption (PSA) techniques have been refined to some degree, PSA still suffers certain disadvantages inherent in being a cyclic process. For example, in the process of separating a strongly adsorbed component from a weakly adsorbed product component of a gaseous mixture, the purge step of the PSA cycle serves the desirable function of removing the strongly adsorbed component from the sieve, but is also accompanied by an undesirable loss of the product component which is contained in the interparticle voids of the bed. The interparticle voidage of a typical adsorbent bed is about forty percent of the total bed volume and, therefore, losses from this source can be significant.
The problem is substantially alleviated and overall performance of the PSA process markedly improved in accordance with the present invention by combining certain percentages of fine particulate kinetically-selective molecular sieve with conventional coarse particles thereof to achieve an optimum volume ratio of comparatively coarse and fine kinetically-selective sieve particles in the bed. A very significant enhancement in yield can be achieved by using these beds in PSA processes, such as nitrogen enrichment.
It is known to combine in a vessel coarse and fine particles intended for adsorption of a material. Ma, U.S. Pat. No. 3,757,490, discloses such a particle mix in a system intended for solid-liquid chromatographic separations. The particles utilized by Ma are all active adsorbent particles and are relatively close in size range in that ninety percent by weight have a diameter within ten percent of the average diameter of all particles. Ma is also concerned only with a solid-liquid system which is markedly different from a PSA gas separation system. Most important, the particles of adsorbent in Ma's system are equilibrium selective as opposed to kinetically selective as will be discussed below.
More recently, Greenbank in European Patent 0 218 403 discloses a dense gas pack of coarse and fine adsorbent particles wherein the size of the largest fine particles is less than one-third of the coarse particles and sixty percent of all particles are larger than sixty mesh. Although not specifically stated, it is evident from the examples that these percentages are by volume. This system is designed primarily for enhancing gas volume to be stored in a storage cylinder. It is mentioned, however, that it can be utilized for molecular sieves. There is nothing in this application, however, which would give insight into the fact that significantly enhanced PSA efficiency could be obtained by combining coarse and fine particles of kinetically-selective sieve material in a single bed. It has been found in accordance with the present invention that, within certain limits as will be defined, a mixture of coarse and fine kinetically-selective sieve particles will unexpectedly give enhanced PSA performance.